Miniaturized devices have been suggested for effecting various mechanical functions. Such devices are fabricated utilizing the techniques of integrated electronic circuit production, as described, for example, in an article "Silicon as a Mechanical Material" by K. E. Petersen, Proc. IEEE 70, 420-457 (May 1982). One field of application of the device components is gas chromatography, in which chromatographic columns, injectors, valves and detectors are integrated into a device formed primarily of laminated silicon wafers. The reference discloses a hot wire detector in which the thin film heater is supported by a silicon membrane in a chamber duct of flowing gas. It is necessary that membranes utilized in such devices have good rigidity, at least some flexibility and, in some critical areas, high temperature stability.
The reference also describes a solenoid valve. An improved valve for such a system is disclosed in "A Microminiature Electric-to Fluidic Valve" by M. J. Zdeblick and J. B. Angell, Transducers 87, pp 827-829 (1987). The valve utilizes a sealed cavity filled with a liquid. One wall of the cavity is formed with a flexible membrane which can press against a pneumatic nozzle. When the liquid is heated, its pressure increases, pushing the membrane toward the nozzle and turning off the valve.
Silicon nitride is mentioned in the Petersen reference as a masking material, and is suggested for use in strain sensors as disclosed in "Deposition Techniques and Properties of Strain Compensated LPCVD Silicon Nitride Films" by H. Guckel, D. K. Showers, D. W. Burns and C. R. Rutigliano, Technical Digest. IEEE Solid State Sensors (June 2-5, 1986). A problem with silicon nitride is that its coefficient of thermal expansion is about half that of silicon, with a corresponding tendency to stress rupture.
Boron nitride deposited by chemical vapor deposition (CVD) is used as a component material as a mask for etching silicon as taught in "Boron Nitride Mask Structure for X-Ray Lithography" by D. Maydan, G. A. Coquin, H. J. Levinstein, A. K. Sinha and D. N. K. Wang, J. Vac. Sci. Technol 16 (6) 1959 (1979). CVD boron nitride is generally in the form of BN and, due to the chemical process, contains hydrogen. The hydrogenated BN changes stress with time and temperature, becoming very tensive to the point of rupture.
Greater precision is always a goal in producing such devices, particularly for further miniaturization. In the case of fluid flow devices, smoothly rounded channels are important to minimize resistance to flow and allow more precise control over cross sectional areas. However, conventional masking for etchants tends to produce an undesirable form of anisotropic etching, resulting in nonuniformity of etched channels. For example etching of a &lt;100&gt; silicon wafer having a mask of silicon dioxide or boron nitride, results in outside corners being etched with additionally tilted faces from &lt;110&gt; and &lt;310&gt; planes, and inside corners being perpendicular sharp corners. Although smooth channels are depicted in the literature, in practice actually etched channels and cavities are not so smooth. In chromatographic operation sharp inside corners may result in peak distortion caused by flow trapping in a corner.
Therefore, objects of the present invention are to provide an improved laminated structure for use in miniaturized devices, to provide a laminated structure with an improved masking layer, to provide a novel membrane material for laminated structures, to provide improved miniature mechanical devices including a gas flow detector and a gas valve, and to provide a process for forming a novel membrane material on a wafer.
Further objects of the present invention are to provide a novel process for forming a mask on a silicon wafer and to provide an improved mask thereby, to provide an improved boron diffusion layer on silicon usful for masking against certain etchants, and to provide a process that allows favorable anisotropic etching for forming smoothly rounded channels in the silicon surface.